The present invention relates to a monitoring arrangement for vented nuclear fuel elements and more particularly to such a monitoring arrangement in combination with an array of fuel elements in a gas-cooled fast nuclear reactor.
In nuclear reactors of the type contemplated by the present invention, solid nuclear fuel is commonly protected and/or contained by cladding formed from a suitable material, such as metal, which effectively separates the solid nuclear fuel from coolant which is circulated through the reactor core to extract heat from the fuel elements. The cladding thus tends to prevent contamination of the coolant by the nuclear fuel material as well as by fission products formed within the fuel elements during operation of the reactor.
In fast reactors of the type contemplated by the present invention, high pressure coolant is required to extract heat efficiently. Pressure equalization is employed to relieve the fuel cladding of undesirable pressure forces. Pressure equalization is accomplished by interconnecting internal gas spaces of the fuel rods within a fuel element with external coolant fluid in order to vent the substantial amounts of fission gases generated in the fuel elements. It is further desirable or necessary to prevent these fission products from entering the main coolant stream of the reactor. For this reason, fuel elements in gas-cooled fast reactors may employ vent systems wherein a small fraction of the gas coolant flow is diverted from the main coolant stream flowing externally of the fuel elements. The diverted coolant flow acts as a sweep gas and is directed past the fuel element vents to carry the fission products to suitable traps located externally of the reactor core. The fission products are retained in the traps and the diverted coolant acting as a sweep gas is returned to the main coolant stream.
The efficiency or operating condition of the individual fuel elements may be accurately assessed by analyzing or monitoring radiation from the fission product gases which are vented therefrom. Fission gases released from the solid fuel must be transported to the fuel element vents from non-leaking fuel rods and elements (where the cladding is integral) by gaseous diffusion, a very slow process at high gas pressures. Thus, during transport to the vents, radioactivity of the fission gases being vented decreases or decays to relatively low levels. On the other hand, in leaking fuel rods, the fission gases are transported to the fuel element vents by forced convection purging caused by coolant leaking into the affected fuel rods. Convection transport takes place rapidly relative to diffusion transport so that relatively limited decay of radioactivity occurs between the leaking fuel rods and the fuel element vents. Radioactivity in gases being vented from fuel elements containing leaking fuel rods is therefore much greater than in integral fuel elements which do not contain leaking fuel rods. The presence and rate of leakage for fuel rods in the respective fuel elements may thus be determined by fission products vented from the fuel elements.
Accordingly, the gases which are vented from the individual fuel elements may be monitored to provide an indication of the operating condition of the individual fuel elements. However, the core of a gas-cooled fast reactor includes a very large number of fuel elements which are closely arranged in order to achieve efficient reactor operation. Thus, it is particularly difficult to provide a monitoring arrangement for effectively monitoring effluent gases from each individual fuel element.
At the same time, the substantial radiation levels within such reactors tend to produce stresses within the individual elongated fuel elements which commonly result in the fuel elements becoming bowed. This tendency of the fuel elements to become bowed may be further increased by thermal conditions within the reactor.
Accordingly, there has been found to remain a need for gas-cooled fast reactors and the like permitting compact arrangement of large numbers of fuel elements while facilitating efficient interconnection of vents for the individual fuel elements with suitable gas monitoring means.